Apparatus for direct plating on resistive liners

ABSTRACT

An apparatus for direct electroplating of a conductive material, such as copper, on resistive liners or substrates is provided. The apparatus includes an integrated in-situ measuring system to follow the actual progress of the front of the conductive material during plating. Feed-back of this information to a power supply allows for more precise control of the effective current density during plating.

FIELD OF THE INVENTION

The present invention relates to electroplating an electricallyconductive material such as a relatively low resistive metal andespecially copper onto a platable resistive metal barrier layer or stackof layers. More particularly, the present invention relates to anapparatus for directly plating onto the resistive metal without the needof a seed or catalyst layer, and especially without the need of a copperseed layer (even though a thin seed may be present, e.g. about1.ANG.-about 10.ANG.). The present invention makes it possible to form acontinuous and relatively uniform layer by growing a thin film from theedge of the surface to be plated towards its center by controlling theconditions of the current or voltage being applied.

BACKGROUND OF THE INVENTION

The current damascene plating process and especially that for copperrequires a copper seed as a conductive layer on top of the highlyresistive barrier liner which covers the underlying substrate such as apatterned wafer. The continuous miniaturization of ULSI technology willeventually require the elimination of this copper seed layer. Withoutthis conductive seed, an applied current or voltage will drop offdrastically within a short distance from the edge where the electricalcontact is made (as will be described below.) As a result of thisso-called terminal effect, a sufficient overpotential, η, for copperdeposition will only exist near the edge of the substrate and plating isobserved at the edge of the substrate only. When applied current isbased on the total area of the substrate the effective current densityfor the perimeter ring is much higher and as a result burned, powderydeposits may be obtained.

Conventional methods to overcome the terminal effects for thin seedlayers such as low plating current, segmented anode configuration, highcopper concentration and low conductivity (low acid concentration)copper plating baths improve the current distribution and result in amore uniform film thickness. However, these methods apply only in thecase where a sufficient plating overpotential exists over the wholesubstrate surface, from edge to center. For very thin seed layers andmore importantly in the absence of a seed layer, the terminal effect ofthe resistive liner or seed causes such a drastic increase in thepotential of the liner material, U_(m)(r), from the edge (r=r₀) tocenter (r=0) of the wafer, that the overpotential, η, becomes zero at acertain distance from the electrical contact and no further plating canoccur:η=U_(eq,Cu) ²⁺ _(Cu) −U _(m)(r)  (1)with U_(eq,cu) ²⁺ _(/Cu) the equilibrium potential (Nernst potential)for copper deposition.

Copper deposition will proceed when η>0, i.e. when U_(m)(r)<U_(eq,Cu) ²⁺_(/Cu). In the case of thin copper seeds (5-50 nm), the sheet resistanceis still low enough to ensure deposition over the whole substratesurface, although with a non-uniform growth rate in the case of aprimary current distribution. In contrast, in the case of highlyresistive liner, the drop-off in the overpotential is much more severeand becomes zero at a certain distance, x=r₀−r, from the edge of thewafer. In this case, deposition is only observed at the edge of thewafer. Additionally, too low current or overpotential results in a lowdensity of nucleation sites leading to powdery, poorly adherentdeposits.

As with copper, the principle of seedless plating holds for thedeposition of any conductive material (metal, compound, alloy,composite, semi-metal or semiconductor) onto a resistive substrate.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for electroplating aconductive material, such as copper, on a liner or substrate. Theapparatus includes:

(a) at least one auxiliary electrode, wherein the at least one auxiliaryelectrode provides a counter electrode to the liner or substrate,wherein the liner or substrate acts as a first electrode;

(b) a programmable power supply providing for the generation of acurrent between the at least one auxiliary electrode and the liner orsubstrate, allowing for the conductive material to be electroplated ontothe liner or substrate, and further providing for the adjustment of thecurrent as a function of the change in the area of the conductivematerial as it is electroplated on the liner or substrate;

(c) a measuring device to detect the propagation of the front of theelectroplated material over the surface of the liner or substrate; and

(d) a computer to process the output of the measuring device andcalculate a new current to be applied by the programmable power supplyas a function of the output of the measuring device.

The measuring device of the apparatus is not limited and can include oneor more reference electrodes, a light source (such as a laser orlight-emitting diode) and at least one photo detector (such as aphotodiode) to measure the reflectivity of the at least one lightsource, an alternating current or voltage generator and analyzer (suchas a frequency response analyzer (FRA) or Lock-in amplifier) to measurethe electrochemical impedance of the system, and/or an alternatingelectromagnetic field generator and sensor for Eddy-currentmeasurements.

The present invention further relates to methods of using the apparatusof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will become apparentupon consideration of the following detailed description of theinvention when read in conjunction with the drawings, in which:

FIG. 1 shows the calculated change in overpotential, η, (calculatedusing the Bessel function assuming a linear change in current) forcopper seeds (FIG. 1A) and platable liners (FIG. 1B) with differentsheet resistance values as a function of distance from the center of awafer;

FIG. 2A shows a set of conventional electronic components, whichsymbolically represents the path of current flow from an electricalcontact with the wafer (cathode) to the opposite electrode (anode), forplating a certain spot on a wafer;

FIG. 2B shows the difference in resistive path for different spots on awafer in a conventional cup or fountain plater;

FIG. 3 shows a schematic representation of the progressive growing ofcopper from the edge to the center of a wafer with an associatedincrease in current;

FIG. 4 shows a schematic representation of a basic two-electrode setupfor direct plating on a wafer with a platable resistive metal layerstack in contact with the plating bath, using a programmable powersupply controlling the current and voltage between the wafer (cathode)and an auxiliary electrode (anode);

FIG. 5 shows a schematic representation of an advanced setup for directplating allowing the measurement of the advancement of the plated metalfrom edge to center, and feed-back to a computer to control the currentand voltage accordingly;

FIG. 6 shows a schematic representation of a basic three-electrode setupfor direct plating on a wafer with a platable resistive metal layerstack in contact with the plating bath, using a programmable powersupply that can control the current between the wafer (cathode) and anauxiliary electrode (anode), and can measure and control the voltagebetween the wafer and the reference electrode;

FIG. 7 shows the plating potential between a rectangular strip (3 cmwide, 9 cm long) with a resistive TaN/Ru liner (with electrical contactmade on one end of the 9 cm long strip) and a saturated mercury-mercurysulfate reference electrode during direct plating of copper (constantcurrent of 0.123 A) for in-situ monitoring of the copper frontprogression;

FIG. 8 shows a schematic representation of a plating potentialmeasurement using an array of reference electrodes (FIG. 8A), areference electrode moving over the wafer surface (FIG. 8B), and anexpected plating potential response (FIG. 8C);

FIG. 9A shows a schematic representation of an experimental setup usedto demonstrate the concept of light reflectivity as means of measuringthe advancement of a plated copper front;

FIG. 9B shows an example of the photo-voltage response during directplating of copper on a resistive Ru layer;

FIG. 10 shows a schematic representation of a light reflectivitymeasurement using an array of LED's and photo diodes (FIG. 10A), asingle set of LED and photo diode moving over a wafer surface (FIG.10B), and an expected photo-voltage response (FIG. OC);

FIG. 11 shows the real and imaginary components of the measuredelectrochemical impedance, Z_(real) and Z_(imag), during direct copperplating on a rectangular strip (3 cm wide, 9 cm long) with a resistiveTaN/Ru liner stack (with electrical contact made on one end of the 9 cmlong strip), illustrating the concept of AC impedance measurements forin-situ monitoring of the copper front progression (FIG. 11A shows thechange Of Z_(real) and Z_(imag) with time and FIG. 11B shows thevariation of Z_(real) and Z_(imag) with plated copper area according toone embodiment of the invention);

FIG. 12 shows a flow-chart representing a concept of the invention wherethe out-put signal of the measuring device is directly used foradjusting the current through a feed-back loop and for detection of thepoint of complete surface coverage; and

FIG. 13 shows a flow-chart representing a concept of the invention wherethe out-put signal of the measuring device is used for in-situmonitoring of the copper front progression and calculation and thecurrent to be applied.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Published Application No. 2004/0069648 A1, the entire disclosure ofwhich is incorporated herein by reference, discloses a method for thedirect electroplating of a relatively low resistive metal, such ascopper, on a resistive substrate. That method allows plating of copperdirectly on at least one liner layer, where the liner layer(s) act as adiffusion barrier for copper into the dielectric. These diffusionbarrier layers typically have a sheet resistance, R_(s), which can beseveral orders of magnitude higher than for currently used copper seedlayers. For example, a typical diffusion barrier layer sheet resistancemay be in the range of 5 to 300Ω/square, whereas copper seed layers mayhave a resistance in the range of 1 to 3Ω/square.

The method disclosed in U.S. Published Application No. 2004/0069648 A1can be described as direct plating or seedless plating. This method isbased on the fundamental concept that the driving force for plating,known as the overpotential, η, becomes non-existent or nil at a certaindistance from where electrical contact is made, the electrical terminal.For back-end processing of silicon wafers, electrical contact istypically made around the edge of a wafer, using multiple pins or asealable ring contact. Accordingly, in such processing, as the distancefrom a wafer edge increases, the overpotential can be expected todecrease. In direct or seedless plating, this overpotential decreasegenerally occurs to a far greater extent than in conventional plating.This effect is the direct result of the severe potential drop (alsoknown as IR-drop and terminal effect) in the resistive liner. FIG. 1shows the calculated change in the overpotential (calculated using theBessel function assuming a linear change in current) for copper seeds(FIG. 1 a) and platable liners (FIG. 1 b) with different sheetresistance values.

Even for very thin copper seeds, a plating overpotential exists in thecenter and edge of the wafer. However, the thinner the copper seed, thegreater the expected potential drop from the edge to the center of thewafer. Such potential drop typically results in a strong non-uniformpotential or current distribution over the wafer surface, but platingoccurs everywhere at the wafer surface, even though at different rates.Many new plating stations have one or more hardware solutions to dealwith the terminal effect and provide more uniform current distributions.All of these solutions basically provide different ways to compensatefor the high resistance of the thin seed.

The path of the current flow from the electrical contacts with the wafer(cathode) to the opposite electrode (anode) in the plating cell can berepresented by a set of electronic components, such as resistors andcapacitors. FIG. 2 a shows the equivalent circuit for plating a certainspot on a wafer and FIG. 2 b shows the difference in resistive path fordifferent spots on the wafer in a conventional cup or fountain plater.The capacitor shown in FIG. 2, which represents the double layer orso-called Helmholtz layer at the electrode/electrolyte interface, onlyplays a role during pulse plating or methods using alternating current(AC). By way of comparison, in direct current (DC) plating, only theresistors are of importance to understand the distribution of thecurrent. In such case, the applied voltage is distributed as follows:V _(applied) =η+I(R _(substrate) +R _(solution))  (2)

The difference in plating overpotential for a spot on the wafer close tothe electrical terminal (current path 1 in FIG. 2 b), and a spot closerto the wafer center (current path 2 in FIG. 2 b), is the result of theadditional large resistance of the substrate in the latter case. Ahigher substrate resistance can be expected to result in morechallenging processing conditions with regard to both overpotential andcurrent distribution. Notably, the current will be lower in a regioncloser to the center of the wafer (smaller overpotential), whichslightly counteracts the drop in potential, which is also dependent oncurrent.

One method of counteracting the effect of increased substrate resistanceresides in making the solution resistance substantially high so that thechange in substrate resistance has little or no effect on the potentialdrop, thereby allowing the overpotential to be essentially the same fromedge to center. This concept forms the basis for one technique foruniform current distribution.

One method of increasing the solution resistance involves lowering itsionic strength or conductivity by using an electroplating bath with lowacidity. In this regard, see, H. Deligianni et al. “Model of waferthickness uniformity in an electroplating tool, ElectrochemicalProcessing in ULSI Fabrication and Semiconductor/Metal Deposition II”,Proceedings of the International Symposium (Electrochemical SocietyProceedings Vol. 99-9) 83-95 (1999), the entire disclosure of which isincorporated herein by reference. However, such a method may not besufficient for very thin copper seeds and, in addition, such a methodtypically requires hardware modifications. Another method involvesartificially increasing the solution resistance by using a plater thatcomprises a porous resistive element placed between a cathode and ananode. “EREX” from Ebara Corporation, is an example of a device thatperforms this method.

Other methods involve using segmented anodes to modify the current flowdistribution In this regard, see, for example, U.S. Pat. No. 5,156,730to Bhatt et al., U.S. Pat. No. 6,497,801 to Woodruff et al., U.S. Pat.No. 6,660,137 to Wilson et al., and U.S. Pat. No. 6,773,571 to Mayer etal., the entire disclosures of which are incorporated herein byreference. Even though these methods tend to be more complex, they alsorely on the solution resistance where the path of least resistance isdetermined by the distance from the multiple anodes to different spotson the cathode, as well as the polarization of each anode.

Thieves may also be used to provide increased center to edge uniformity.In this regard, see S. Mehdizadeh et al., “Optimization ofElectrodeposit Uniformity by the Use of Auxiliary Electrodes”, J.Electrochem. Soc., Vol. 137, No 1, p. 110-116, (January 1990), theentire disclosure of which is incorporated herein by reference.

In all of the above approaches, different kinds of hardware are used inan attempt to create uniform current distribution by correcting thechange in overpotential along the surface. Notably, this approachassumes that the overpotential can be described by a continuousfunction. However, when the substrate resistance is large enough tocause the overpotential to drop to zero, the description of theoverpotential along the wafer surface becomes discontinuous. In suchcase, correction of the effective series resistive by hardware becomesextremely difficult.

U.S. Published Application No. 2004/0069648 A1 provides a method fordirect plating, which can overcome at least one of the challengesdiscussed above, including the situation where the overpotential alongthe wafer surface can be described as discontinuous. The methoddisclosed therein imparts a technique for plating across a wafer, havinga thin liner only, from the edge towards the center; i.e the wafercoverage is not instantaneous as with conventional techniques butchanges with time. This method applies a current waveform, whichaccounts for the change in plated copper area and is illustratedschematically in FIG. 3. The operating principle of this method involvesapplying a continuous current or voltage ramp (or multiple current orvoltage steps approaching continuous change) to keep the effective localcurrent density constant within a certain range. When the effectivelocal current density needs to be controlled accurately, a polynomialfunction for the current can be applied, which can be determined bymeasuring the progression rate of the plated front from the edge to thecenter ex-situ.

The present invention relates to an apparatus that can perform themethod disclosed in U.S. Published Application No. 2004/0069648 A1.Specifically, the present invention relates to an apparatus for directplating on resistive liners having an integrated in-situ measuringsystem to follow the actual progress of a plated front during plating.Feed-back of this information to the power supply allows for moreprecise control of the current waveform to provide a constant localeffective current density during plating of the trench and via features.

The present invention is not limited to any specific type of platingapparatus, and includes, for example, cup and/or fountain platers(“Equinox” from Semitool and “Sabre” from Novellus), thin cell platers(“Slim cell” from AMAT and “EREX” from Ebara) and paddle cells (IBM). Inaddition, the present invention relates to plater configurationsindependent of cell volume, anode configurations (such as separatedanodes or virtual anodes), diffusers for current distribution (resistiveand not resistive), thiefs, flow, rotation, and/or solution chemistry.

The current densities necessary to achieve plating in an apparatuscorresponding to the invention are not limited and can cover a verybroad range. The current densities may, for example, range from about 10μA/cm² to about 2 A/cm², depending on the application, the platingprocess, the plated material and the metal ion concentration in theplating bath. The current densities may typically be expected to rangefrom about 0.1 mA/cm² to about 100 mA/cm², such as from about 3 mA/cm toabout 60 mA/cm². The voltage depends on the tool configuration. Whilenot limited, the voltage employed typically ranges from about 0 to about50 volts, such as from about 0 to about 20 volts, or from about 0 toabout 10 volts.

The solution chemistry of the plating bath is not limited and includesall plating bath materials disclosed in U.S. Published Application No.2004/0069648 A1. For example, the plating bath may comprise a coppersalt, optionally containing a mineral acid, and optionally one or moreadditives selected from the group consisting of an inorganic halidesalt, an organic sulfur compound with water solubilizing groups, abath-soluble oxygen-containing compound, a bath-soluble polyethercompound, or a bath-soluble organic nitrogen compound that may alsocontain at least one sulfur atom.

While copper is the primary conductive material contemplated fordeposition, an apparatus falling within the scope of the presentinvention can be used to plate other materials. Examples include allplatable metals, alloys, composites, semiconductors and/or polymers fromaqueous, non-aqueous or mixed electrolyte solutions or melts. Metalsthat can be plated according to processes falling within the scope ofthe present invention may include, for example, those selected from thegroup consisting of Cu, Ag, Au, Pb, Ni, Pd, Co, Pt, Rh, Ru, Cr, Sb, Bi,Sn, In, Fe, Zn, Cd and and alloys, mixtures, and multilayers of thesame. Other metals that may be plated can include, for example, thoseselected from the group consisting of Re, Tc, Os, Ir, Se, Te, Mn, andalloys, mixtures, and multilayers of the same, as well as alloys,mixtures and multilayers of any of the above metals with Al, Mg, W, V,Ti, Ta, Mo, Ce, Ga, Gd, Hf, Zr, La, Y, Sr, Tl, Eu, Dy, Ho and Nb. Inaddition, oxides, sulfides, phosfites, and borides, of any of the abovemetals may be used, as well as any alloys mixtures or multilayers of anyof the above materials with Si, C, Ge, As, O, P, S, and/or B.

Metals that may be plated further include elemental semiconductors suchas Si and Ge, which may further include C, H and/or F, and alloys,mixtures, and multilayers of the same. In addition, materials that maybe plated include: semiconducting oxides, such as ZnO and TiO₂; III-Vsemiconductors such as InAs, InP, InSb and GaS; II-VI semiconductorssuch as CdSe, CdS, CdTe, ZnS, ZnSe, ZnTe; and tertiary and quaternarysemiconductor compounds of combinations above and with Cu, Sr, Ba, andalloys, mixtures, and multilayers of the same. In addition, materialsthat may be plated include: conducting oxides wherein the metal isselected from the group consisting of Cd, Sn, Ga, In, Cu, Ru, Re, andPb, and alloys, mixtures, and multilayers of the same; conductingpolymers, for example, polypyrrole and polyaniline; semiconductingpolymers; and biominerals.

The thickness of the plated metal, while not limited, can typically beexpected to range from about 0.02 microns to about 25 microns, such asfrom about 0.1 microns to about 2 microns, including from about 0.3microns to about 1 micron.

A plating apparatus falling within the scope of the present inventionmay be used in the fabrication of semiconductor wafers, such as Si orGe, with diameters ranging from, for example, about 5 inches (about 120mm) to about 12 inches (about 300 mm). The apparatus may also be usedfor substrates with other than circular geometries. In U.S. PublishedApplication No. 2004/0069648 A1, an example was given for a rectangularshaped substrate with electrical contact made on one edge. In such case,the plated front migrates from the edge, where contact was made, towardsthe opposite edge over time. In such case, in-situ measurement of thecopper front position is also desirable.

The liner or diffusion barrier layer or layers on which the conductivemetal, such as copper, may be plated is not limited. Examples ofmaterials that can be used include tantalum, tantalum nitride, titanium,titanium nitride, tungsten, tungsten nitride, ruthenium, platinum,rhodium, palladium, rhenium, cobalt, molybdenum, iridium, vanadium,chromium, yttrium, zirconium, niobium, hafnium and mixtures, alloys andmultilayers thereof. Further examples include gold, thallium, lead,bismuth, iron, nickel, copper, aluminum, silicon, carbon, germanium,gallium, arsenic, selenium, rubidium, strontium, silver, cadmium, tin,antimony, tellurium, osmium, and mixtures, alloys, and multilayersthereof. The alloys of the above metals can include various alloyingmaterials including, but not limited, to O, S, N, B and P.

A basic configuration of an apparatus relating to the invention ispresented schematically in FIG. 2. This figure shows a resistivesubstrate, such as a silicon wafer with a thin copper barrier liner,incorporated in a standard plating apparatus such as a cup or fountainplater (FIG. 2 b). The resistive substrate is contacted with anelectrical contact, such as a ring contacting the liner at the edge ofthe wafer, and connected to the negative terminal of a programmablepower supply. An anode (consumable or inert) is connected to thepositive terminal of the power supply. The desired current waveform canbe inputted through a computer controlling the power supply.

FIG. 4 can also be interpreted as showing a basic configuration of anapparatus relating to the invention. This figure shows a silicon waferwith a resistive substrate 10, an anode 20, a plating bath 30, and aprogrammable power supply 40. Without the use of a detection method tomeasure the progression rate of a plated front, the progression rate ofa plated front would be expected to be determined ex-situ, for example,by partial plating experiments.

A representation of an example of a device having an integrated in-situmeasuring system to follow the actual progress of a plated front isillustrated schematically in FIG. 5. This figure shows a resistivesubstrate 10, an anode 20, and a plating bath 30. An in-situ measuringdevice 60 is used to determine the area of a plated material 50, such ascopper. This information is then used in a feed-back loop via a computer70 to control the power supply 40. In such a device, relatively precisecurrent control can be achieved and the effective local current densityfor plating each feature at the wafer can be approximately uniform.

In devices falling within the scope of the present invention, thein-situ measurement of the plated copper front is not limited to anyparticular method. Examples of methods that can be used include:potential measurement, optical measurement, AC or impedance measurement,and Eddy current measurement.

Potential Measurement

The potential measurement method is based on the principle that theplating potential will change when the plated copper increases and willreach a certain value when the whole wafer is covered. Measurement ofthis potential provides an indirect method for controlling the appliedcurrent waveform. The potential can be measured between the cathode andanode, for example, as shown in the configuration of FIG. 4. However, inthis configuration, the measured potential difference is the result ofboth the electrode potential at the cathode/electrolyte interface andthe anode/electrolyte interface together with tooling factors such asdistance between anode and cathode, diffusers, ionic strength of theplating solution, and contact resistance. In this case, no distinctioncan be made between changes occurring at the anode and cathode. A morereliable method is to use at least one reference electrode, which has aconstant electrode potential. The potential is measured between the workpiece or wafer (cathode) and the reference electrode. In this case, themeasured potential difference or plating potential is unaffected bychanges at the anode.

FIG. 6, schematically represents a plating tool in which a referenceelectrode 80 is placed in the plating bath 30 in-between the wafer withresistive liner 20 and anode 10. For potential control, a three or fourterminal power supply 41 is now required, where one of the additionalinputs can be used for connecting the reference electrode 80. Examplesof reference electrodes include: Calomel, mercury sulfate, silver and/orsilver/silver chloride. A pseudo reference can also be used as a thirdelectrode. Examples of pseudo reference electrodes include: copper,platinum, or gold wire. Typical electrode potentials for some referenceelectrodes are 0.24V for Saturated calomel electrode (SCE), 0.22V forAg/AgCl electrode and 0.65V for saturated mercury-mercury sulfateelectrode (SMSE). By convention the “Normal hydrogen electrode” (NHE)also called “Standard hydrogen electrode” (SHE) has an electrodepotential of 0V (and is the potential of 1 mole/liter of protons, H⁺, inchemical equilibrium with hydrogen gas, H₂, at 1 atmosphere).

The plating potential (measured potential difference between work pieceand reference) is the sum of the electrode potential difference, U, andthe IR-drop in the solution (plating potential=U+IR). The electrodepotential difference depends on the electrode potential of the reference(see stated values above). A typical range of electrode potentialdifferences, U, for copper plating is about 0.34V to −1V versus NHE;0.1V to −0.76V versus SCE, or −0.31V to −1.65V versus SMSE. The IR-dropdepends on the applied current, solution resistance and the position ofthe reference electrode in the plating cell. Values may range from 0 to12V. The IR-drop is the result of the electric field between the anodeand cathode, and can be minimized by bringing the reference electrode asclose as possible to the substrate surface. In practice, this can bedone with a Luggin capillary. A Luggin capillary, is a thin capillarybrought close to the work piece (wafer surface in our case), to samplethe potential of the working electrode/electrolyte interface withoutinterference of the electric field present between the cathode and anodein the solution.

The electrode potential of a work piece is typically dependent onchanges in the chemical composition of the surface (deposition offoreign material or adsorption of organic molecules) and changes inelectrolyte composition (e.g. concentration changes due to iondepletion) at the measured interface. Thus, in the case of direct copperplating on a liner, such as ruthenium, by sequential surface coverageaccording to the teachings of application Ser. No. 10/269,956, thetransformation of a bare Ru surface towards a complete Cu surface, within-between the co-existence of Cu and Ru with changing ratio in surfacecoverage, is directly measurable by the change in plating potential.

In this regard, see FIG. 7A, which shows the response of the platingpotential during direct copper plating on a narrow Ru strip (3 cm wide,9 cm long silicon/SiO₂ strip with TaN/Ru liner, submersed in a copperplating bath). Electrical contact was made on one end of the strip,allowing copper growth along the 9 cm strip towards the other end. Thecurrent was kept constant at 0.123A (Pt mesh counter electrode, andsaturated mercury-mercury sulfate reference electrode). The platingpotential (U+IR) is about −7V for about 10 seconds and then dropsrapidly to −7.5V at about 50 s and remained constant thereafter. The Rusurface gets fully covered with Cu in about 50 seconds (the progressionof the copper front could be observed visually in the glass cell aswell). From this, a constant copper front progression rate of 0.18cm/second can be estimated. The change in area is 0.54 cm²/second.

FIG. 7B shows the plating potential as a function of plated Cu area,which could be used as a calibration curve according to the invention.In one embodiment of this invention, the plating potential is used forin-situ monitoring of the plated copper front and exact control of thecurrent program. This method requires a calibration curve which is usedby the computer for calculating the current based on the platingpotential output. The calibration curve needs to be determined inadvance by correlation of plating potential with a more directmeasurement of the copper front. This can be done by a combination ofother in-situ monitoring techniques, such as optical measurements asdescribed below, or by ex-situ measurements such as sheet-resistance(Rs) mapping, ellipsometry, chemical analysis, reflectivity, etc. In thelatter case, partial plating experiments allow correlation betweendifferent positions along the plating potential-time curve withexperimentally measured copper areas. Note that the approach describedabove has limitations since the plating potential is an indirect measureof the plated copper area, and thus a more direct approach is desirable.

Since the wafer coverage changes over time, the electric field betweenthe cathode and anode changes over time. The change in electrical fieldcan be measured as changes in the plating potential (through change inthe IR-drop) with several reference electrodes positioned at differentlocations along the wafer radius. Alternatively, at least one fastmoving reference electrode along the wafer radius can be used. Theplating potential measured by at reference electrodes near coppercovered area will be drastically different from those measured near bareliner areas, thus creating a diameter scan or profile of the coppercoverage. In one embodiment of this invention, an array of referenceelectrodes in placed along the wafer radius, and as many volt meters areused to measure the local plating potential. In another embodiment, atleast one reference electrode is moving fast over the wafer surface fromedge to center, to measure the plating potential profile over time.

In this regard, see FIG. 8 showing a schematic illustration of an arrayof reference electrodes 100 (FIG. 8A) and a fast moving referenceelectrode 110 (FIG. 8B) across a partially plated wafer (wafer withresistive liner 10 and plated metal 50), and the expected potentialresponse (FIG. 8C). The plating potential versus wafer radiusinformation provides direct information of the plated metal area andthus allows exact control of the current by feed-back from the computerto the power supply.

In yet another embodiment, the reference electrode may follow the platedcopper front (which is determined by the change-over in platingpotential associated with change from liner, such as Ru, to platedmetal, such as copper). The position of the reference electrode is adirect measure of the plated copper area.

Optical Measurement

When the reflectivity of the substrate and the plated copper issufficiently different, light reflection measurements can be used tofollow the plated copper front. An application of this principle isschematically represented in FIG. 9. FIG. 9A shows a diagram of areflectivity measuring setup. In this figure, a wafer 10 is placed on awafer chip rotator 20. A laser 30 provides a laser beam 40, which, withthe help of mirrors 50, monitors the center for the wafer. The reflectedbeam 60 is collected at a photo-diode detector 70. A change inphoto-voltage from the detector indicates the copper has reached thespot of light aimed at the surface. When a thick film is formed withgood reflectivity, the reflectivity becomes essentially constant.Similar optical methods have been proposed for to monitor the removalrate of a metallization during CMP, see, for example, U.S. Pat. No.6,707,540 to Lehman et al., the entire disclosure of which isincorporated herein by reference.

FIG. 9B shows the results of a simple experiment using a single spot inthe center of a wafer according to the setup in FIG. 9A. In thisexperiment, a wafer piece measuring 4 centimeters by 4 centimeters and acopper plating bath was used and a constant current of about 80 mA wasapplied. As shown in FIG. 9B, the photo-voltage increased drasticallywhen the copper reached the center of the wafer piece.

In an actual plating tool, several spots at the wafer may be illuminatedthrough the use of several small light emitting devices (LED's) with thereflectivity monitored with a photo diode array. Alternatively, as forthe plating potential method, at least one set of a LED and photo diodecan be used, which is scanned rapidly over the wafer surface to measurethe reflectivity as a function of wafer radius or track the copper frontby positioning itself where the reflectivity difference is the largest(border between bare substrate, such as ruthenium liner, and platedmetal, such as copper).

In this regard, see FIG. 10, showing a schematic illustration of anarray of LED and photo diodes 120 (FIG. 10A) and a one set of fastmoving LED and photo diode 130 (FIG. 10B) across a partially platedwafer (wafer with resistive liner 10 and plated metal 50), and theexpected photo voltage or reflectivity (FIG. 10C). The reflectivityversus wafer radius information provides direct information of theplated metal area and thus allows exact control of the current byfeed-back from the computer to the power supply.

In another embodiment, the reflectivity of the total substrate area canbe measured, with current adjustments made as a function of thismeasurement. In this case the total measured reflectivity is a linearfunction of metal coverage and thus plated area, and the current isdirectly proportional with the measured reflectivity. For example in thecase of direct copper plating on Ru, the reflectivity of Cu is higherthan the Ru substrate. The maximum change in reflectivity (100%)corresponds to the difference in photo-voltage of a complete Cu surface,U_(photo,Cu), and a complete Ru surface, U_(photo,Ru). The totalreflectivity, R(%), of a surface can be defined as:R(%)=100×[V _(photo,t)−(V _(photo,Cu) −V _(photo,Ru))]/(V _(photo,Cu) −V_(photo,Ru)),with V_(photo,t) the photo-voltage measured at time, t. When thereflectivity is proportional to the plated copper area, then the currentis directly controlled by the determined reflectivity.

In the case of direct copper plating for back-end of line semiconductorprocessing, the semiconductor wafer itself can be used as photo-detector(since semiconductors form the bases of photo-diodes). The back of thewafer can be connected to a voltmeter, which is in communication with acomputer to control the power supply.

In addition, other optical techniques such as light absorption, lightscattering, or light emission (if the structure has light emittingproperties) can be similarly used

AC Measurement

Another method of monitoring a plated metal front in an apparatusfalling within the scope of the present invention involves AC orimpedance measurements by superposition of a small alternating currentor voltage to the applied current or voltage for plating by use of asinusoidal signal generator. Such alternating voltage or currentgenerates a respective alternating current or voltage responsedetermined by the electrochemical impedance of the substrate/electrolyteinterface. The electrochemical impedance can be measured using a Lock-inamplifier or Frequency response analyzer (FRA) by comparing the inputand output sinusoidal signals which differ in amplitude and phase. Theimpedance vector (defined by its magnitude, /Z/ and phase angle, φ) canbe represented by its real part, Z_(real) (representing the resistanceof the interface) and an imaginary part, Z_(imag) (representing thecapacitance of the interface). The measured electrochemical impedancedepends strongly on the frequency of the applied sinusoidal signal. Atvery high frequencies (1 MHz-100 kHz), the series resistance of thesystem (electrical connections, wires, contact resistance and solutionresistance) is measured (typically between 1 to 1000 ohms, depending ontool design and setup). The series resistance contains no information ofthe substrate/electrolyte interface and is therefore not suitable forthis application. At very low frequencies (100 Hz-1 Hz), diffusionprocesses may determine the measured impedance (known in the art as theWarburg impedance), which is also independent of substrate and thereforenot suitable for our application. In the intermediate frequency range(10 kHz-100 Hz), both the capacitance of the substrate/electrolytedouble layer (known as the Helmholtz layer in the art, see FIG. 2A), andthe resistance associated with the charge transfer over theelectrode/electrolyte interface (R_(ct), as in FIG. 2A) determine themeasured impedance.

The electrochemical impedance for the bare substrate/electrolyteinterface will be different than that for the interface of theelectrolyte with a plated metal such as copper. Values can, for example,range from about 0.1 ohm cm² to about 1000 ohm cm² for the resistanceand about 10 μFarad/cm² to about 100 μFarad/cm², depending on appliedcurrent and potential. Since the impedance depends on the surface area,the total measured impedance will be a function of the plated copperarea. As with other methods described elsewhere herein, suchmeasurements can be used in a closed loop to control the current programapplied.

In this regard, see FIG. 1I A, which shows an example of the real andimaginary part of the impedance, measured in-situ during direct platingon a long silicon strip with a resistive Ru film on top (3 cm wide, 9 cmlong strip with a resistive TaN/Ru liner stack and electrical contactmade on one end of the 9 cm long strip, constant applied current of0.135 A). The real part of the impedance, Zeal, increases linearly withtime and reaches a somewhat constant value around 50 seconds, coincidingwith the point of complete copper coverage. The imaginary part of theimpedance, Z_(imag), is constant up to about 35 seconds and then changesrapidly to reach a plateau again around 50 seconds. Since the copperarea increases with time (linearly for a rectangular strip), and since,in this experiment, the current was kept constant, the current densitydecreased linearly with time.

Z_(real) and Z_(imag) are plotted as a function of plated Cu area inFIG. 11B. The change in Z_(real) is a direct measurement of the changein current density (Z_(real) is inversely proportional with currentdensity). Hence, it is clear to a person in the art, that Z_(real) canbe used to directly monitor and control the current density at Cu. Forexample, in this case, keeping Z_(real) constant by adjusting thecurrent through a feed back loop is one way to control the currentdensity with impedance measurements.

A flow diagram for a feed-back loop working according to theseprinciples, is shown in the fist part of FIG. 13. In addition, Z_(imag)can be used for controlling full coverage of the wafer or sample. Thesharp change in Z_(imag) indicates the Cu coverage is coming tocompletion, and can be used as end-point detection. From that point on,conventional plating steps can be applied. The end of feed-back loopcontrol is also incorporated in the flow chart of FIG. 13.

An embodiment for controlling the current density during wafer coveragestep, using impedance measurement method is shown in FIG. 13. An initialconstant current or voltage may be applied for a certain time to grow acopper ring around the edge. During or at the end of this time theimpedance (Z) is measured. The subsequent applied currents, I(t) and/orvoltage, V(t), are controlled by a feed-back loop and change with timein order to keep the real part of the impedance, Z_(real), constant(dZ_(real)/dt=0); i.e. equal to the initially measured value. Thisfeed-back loop is maintained until a drastic change in the imaginarypart of the impedance is observed (dZ_(imag)/dt>>0). After this step,the wafer is completely covered with a continuous copper film, andconventional plating steps may be applied for then. For example, fromthat point on the current may be kept constant for a certain time toallow the copper thickness to grow, and small damascene features tofill.

Eddy Current

A somewhat analogous method to the AC measurement method involves themeasurement of Eddy currents. Eddy currents are small currents near thesurface of a wafer or substrate that exist as the result of an appliedalternating magnetic field. Eddy currents are a function of thefrequency and the magnetic flux as well as the resistivity andpermeability of the surface. Since the surface gradually changes duringplating, measured Eddy currents can be used to determine the platedcopper area. As with other methods described elsewhere herein, suchmeasurements can be used in a closed loop to control the current programapplied. Eddy-current measurement methods are well known in the art ofchemical mechanical polishing (CMP) and are typically used for end-pointdetection. In this regard, see for example, U.S. Pat. No. 6,072,313 toLi, et al. and U.S. Pat. No. 6,707,540 to Lehman, et al., the entiredisclosures of which are incorporated herein by reference. In addition,see U.S. Pat. No. 4,556,845 to Strope, et al., the entire disclosure ofwhich is incorporated herein by reference, which describes Eddy-currentmeasurements for in-situ thickness evolution of an electroless deposit(i.e. formed without applied current or voltage).

Each of these methods is based on the measurement by a sensor of acurrent (Eddy current) induced in the film by an alternatingelectromagnetic field, and the sensor signal is dependent on the filmthickness. Similar approaches can be used for monitoring the platedmetal front. Similar to the reference electrode arrays and photodiodearrays used for the potential and optical methods, an array of coils forinducing an alternating magnetic field and Eddy current measurement canbe used. Alternatively, a set of coils for Eddy-current measurements canbe moved over the diameter of the surface to measure the thicknessvariation (with respect of before plating) for determination of theplated metal front.

FIG. 13 shows a schematic and flow-diagram for an embodiment usingmethods which measure the time dependent copper coverage, such as thepotential method, the optical method and the Eddy current method. Aninput signal (such as a light beam) may be applied by the measuringinstrument, which induces an output signal (such as a reflected lightbeam) measured by the measuring instrument. Alternatively, no externalinput signal is required, but an output signal (such as platingpotential) is still measured. The output signal is a measure for theplated copper area, which is calculated by the computer. For thiscomputation, a calibration curve may be required. The computercalculates the current according to the determined plated area (Current(I)=Area (A)×current density (i)), and communicates this current to thepower supply. This process continues until full wafer coverage isachieved. At this point, the current program can continue usingconventional plating steps.

While the present invention has been described in an illustrativemanner, it should be understood that the terminology used is intended tobe in a nature of words or description rather than of limitation.Furthermore, while the present invention has been described in terms ofseveral illustrative embodiments, it is to be appreciated that thoseskilled in the art will readily apply these teachings to other possiblevariations of the inventions.

1. An apparatus for electroplating a conductive material on a resistiveliner or substrate comprising: (a) at least one auxiliary electrode,wherein the at least one auxiliary electrode provides a counterelectrode to the liner or substrate, wherein the liner or substrate actsas a first electrode; (b) a programmable power supply providing for the:(i) generation of a current between the at least one auxiliary electrodeand the liner or substrate, allowing for the conductive material to beelectroplated onto the liner or substrate; and (ii) adjustment of thecurrent as a function of the change in the area of the conductivematerial as it is electroplated on the liner or substrate; (c) ameasuring device to detect the propagation of the front of theelectroplated material over the surface of the said resistive liner orsubstrate; and (d) a computer to process the output of the measuringdevice and calculate a new current to be applied by the programmablepower supply as a function of the output of the measuring device.
 2. Theapparatus of claim 1, wherein the measuring device comprises at leastone reference electrode.
 3. The apparatus of claim 2, wherein thereference electrode is selected from the group consisting of Calomel,mercury sulfate, silver and silver/silver chloride.
 4. The apparatus ofclaim 1, wherein the measuring device comprises an array of referenceelectrodes positioned along the substrate surface.
 5. The apparatus ofclaim 4, wherein the reference electrodes are selected from the groupconsisting of Calomel, mercury sulfate, silver and silver/silverchloride, and combinations thereof.
 6. The apparatus of claim 1, whereinthe measuring device comprises at least one light source and at leastone photodiode to measure the reflectivity of the at least one lightsource.
 7. The apparatus of claim 6, wherein the light source comprisesa laser or an array of light emitting diodes.
 8. The apparatus of claim1, wherein the measuring device comprises an alternating current orvoltage source.
 9. The apparatus of claim 8, wherein the measuringdevice further comprises an impedance analyzer selected from the groupconsisting of a frequency response analyzer and a lock-in amplifier. 10.The apparatus of claim 1, wherein the measuring device comprised atleast one Eddy-current inducing and sensing device.
 11. The apparatus ofclaim 1, wherein the conductive material comprises copper.
 12. Theapparatus of claim 1, wherein the liner or substrate comprises aresistive metal selected from the group consisting of tantalum, tantalumnitride, titanium, titanium nitride, tungsten, tungsten nitride,ruthenium, rhenium, cobalt, molybdenum, chromium, indium, platinum,gold, thallium, lead, bismuth, vanadium, cobalt, iron, nickel, copper,aluminum, silicon, carbon, germanium, gallium, arsenic, selenium,rubidium, strontium, yttrium, zirconium, niobium, rhodium, palladium,silver, cadmium, tin, antimony, tellerium, hafnium, and osmium, andmixtures, alloys, and multilayers of the same.
 13. The apparatus ofclaim 1, wherein the current has a density ranging from about 10 μA/cm²to about 100 mA/cm².
 14. The apparatus of claim 1, wherein theconductive material has a final thickness ranging from about 0.3 micronsto about 10 micron.
 15. A method for electroplating a conductivematerial on a liner or substrate comprising using the apparatus ofclaim
 1. 16. The method of claim 15, wherein the measuring devicecomprises a reference electrode.
 17. The method of claim 16, wherein thereference electrode is selected from the group consisting of Calomel,mercury sulfate, silver and silver/silver chloride.
 18. The method ofclaim 15, wherein the measuring device comprises at least one lightsource and at least one photodiode to measure the reflectivity of the atleast one light source.
 19. The method of claim 18, wherein the lightsource comprises a laser or an array of light emitting diodes.
 20. Themethod of claim 15, wherein the measuring device comprises analternating current source.
 21. The method of claim 20, wherein themeasuring device further comprises an analyzer selected from the groupconsisting of an impedance analyzer and a lock-in amplifier.
 22. Themethod of claim 15, wherein the measuring device measures anEddy-current.
 23. The method of claim 15, wherein the conductivematerial comprises copper.
 24. The method of claim 15, wherein the lineror substrate comprises a resistive metal selected from the groupconsisting of tantalum, tantalum nitride, titanium, titanium nitride,tungsten, tungsten nitride, ruthenium, rhenium, cobalt, molybdenum,chromium, indium, platinum, gold, thallium lead, bismuth, vanadium,chromium, cobalt, iron, nickel, copper, aluminum, silicon, carbon,germanium, gallium, arsenic, selenium, rubidium, strontium, yttrium,zirconium, niobium, rhodium, palladium, silver, cadmium, tin, antimony,tellerium, hafnium, and osmium, and mixtures, alloys, and multilayers ofthe same.
 25. The method of claim 15, wherein the current has a densityranging from about 10 μA/cm² to about 100 mA/cm².
 26. The method ofclaim 15, wherein the conductive material has a final thickness rangingfrom about 0.3 microns to about 1 micron.